In the past several years MIC baluns formed by a pair of coupled transmission lines have been used as four port circuits, with the unbalanced input terminal on the same end as the ground terminal, and the third and fourth terminals emitting two balun output signals which are 180.degree. out of phase. It is difficult to achieve output signals of equal amplitude in prior art baluns, especially when the bandwidth is an octave or more Additionally, when the length of a prior art balun becomes an integral multiple of even mode half-wavelengths at select frequencies in-band, problems are incurred in the amplitude properties observed at the balun outputs due to extreme over coupling.
If such a prior art balun is used in a mixer or modulator that operates at a frequency where these effects occur, then the conVersion loss, isolation and spurious performance show a troublesome discontinuity. Performance of the prior art balun is optimized when the prior art balun uses suspended broadside coupled strip lines to achieve maximum coupling. Such baluns still fall short of having equal amplitude outputs, and they also require double sided circuitry with two transmission lines, one on each side of an extremely thin substrate.
Another type of prior art balun uses an edge-coupled pair of transmission lines which are on the same plane as the top surface of the substrate. Adequate coupling is even more difficult to achieve with this type of prior art balun, and the useful band width is much narrower. The functionality of this type of prior art balun is reduced even more if a ground plane is brought near the backside of the substrate. Consequently, these coupled microstrip baluns are seldom used because of the enhancement of the even mode of propagation and the extreme amplitude inequality and phase inaccuracy at the balun outputs.
Other types of prior art baluns such as coaxial and slot line baluns using a four port hybrid, or circuits containing magic-tees, still have band width constraints because of amplitude inequalities between balun output signals and because of resonance problems which come about because of circuit topology and line lengths. The prior art baluns that function most successfully tend to involve low dielectric constant material such as polytetrafluroethylene-type, or PTFE, materials which are physically flexible.
The drawback to the use of these materials is that one must solder such assemblies together and the construction of circuits built in this way becomes a complicated, costly and time consuming art, rather than a science of mass production with repeatable results. Regardless of the material type used, these prior art baluns are forced to be extremely thin to achieve proper coupling. As a result, mechanical reliability becomes questionable when these baluns are used under various environmental conditions. Additionally, mixers and modulators which use these prior art baluns typically see these baluns as reflective loads. This gives rise to large efficiency and performance variations.